This invention relates generally to an improved stem-type prosthesis and more particularly relates to a stem-type femoral prosthesis particularly useful in artificial hip joints and of the type including a head, neck and stem, and in some embodiments a collar and still more particularly relates to an improved extended neck stem-type prosthesis.
Stem-type femoral prostheses generally include a spherical head, neck and stem and are generally fixtured into the medullary cavity of the proximal end of the femur by the use of cement as described in U.S. Pat. No. 4,021,865 issued May 10, 1977 to John Charnley or by use of biological fixation into porous surfaces as described in U.S. Pat. No. 4,156,943 issued June 5, 1979 to John P. Collier and in U.S. Pat. No. 3,808,606 issued May 7, 1974 to Raymond G. Tronzo or by direct insertion into the proximal femur without the use of cement or a porous or roughened surface as described in U.S. Pat. No. 4,031,571 issued June 28, 1977 to Gunther Heimke et al.
In U.S. Pat. No. 4,141,088 issued Feb. 27, 1979 to James T. Treace there is described a hip prosthesis wherein the femoral stem prosthesis uses a neck portion whose cross-sectional area is narrower in its anterior-posterior dimension than in its medial-lateral dimension (note FIGS. 1 and 2). The use of non-circular neck cross-section for the purpose of improving the range of motion of the artificial joint is an important attribute because of significant problems of the dislocation of a joint resulting from inadequate motion range. The Treace patent also describes a technique wherein prostheses with varying neck length are made so that the distance from the center of the head to a line passing along the lateral border of the stem or the stem axis is held constant so that a specified anatomical distance is always maintained.
The head of the femoral prosthesis, as noted above, is generally spherically shaped and articulates either with the natural acetabulum or with an acetabular replacement such as disclosed in U.S. Pat. No. 3,722,002 issued May 27, 1973 to John Charnley, U.S. Pat. No. 3,829,904 issued Aug. 20, 1974 to Robin Ling et al., and U.S. Pat. No. 3,848,272 issued Nov. 19, 1974 to Douglas G. Noiles or U.S. Pat. No. 3,863,273 issued Feb. 4, 1975 to Robert G. Averill and U.S. Pat. No. 3,982,281 issued Sept. 28, 1976 to Richard P. Giliberty. This articulation occurs during walking and other normal human activities during which loads are transmitted across the hip joint. Thus, load must be transferred from the proximal femur to the pelvis. In particular, loads transmitted by the acetabulum or acetabular component to the spherical head of the femoral prosthesis must be transmitted by the neck to the stem. In addition, many prostheses employ a calcar collar so that load may be transmitted through the neck and collar directly to the calcar in addition to the transmittal of load by, the stem through the intramedullary canal to the femoral shaft. Collars are generally of two types: those such as shown in the Heimke et al. patent where the collar is distinct from the neck and those as shown in the Charnley and Treace patents where the collar is simply the inferior surface of the neck region. Typical of the collarless design is the stem disclosed in U.S. Pat. No. 4,310,931 issued Jan. 19, 1982 to Muller.
In general, the direction of the resultant joint reaction load on the head of the femoral component is not coincident with the locus of the centroids of the cross-sections of the neck. As the result, joint loading generally introduces bending and shear loading on the neck in addition to the compressive load which would result were the force vector coincident with the centroid locus; as a result, bending, shear and compressive stresses exist in the neck. As used herein, a neck axis is defined as a line connecting the centroid of the cross-section of the most proximal region of the neck with a centroid of a cross-section of the most distal region of the neck where such centroids include only the neck regions and are taken perpendicular to the neck axis. Generally, the axis of the neck is oriented relative to the joint reaction load so that for most of the loading conditions encountered during walking and other activities, significant bending loads are applied to the neck such that the lateral surface of the neck encounters tensile stresses. Such tensile stresses are particularly harmful since fatigue fracture is primarily a tensile phenomenon wherein a local imperfection produces a stress magnification effect which results in the initiation of a crack. The crack tip itself then is a stress riser producing the stress magnification and the continuing removal and application of load as experienced during human activity can produce slow propagation of this crack until catastrophic failure of the neck occurs. Shearing loads can also produce tensile stresses, and the noted fatigue fracture can result from the application of shearing loads. Compressive loading, however, cannot produce tensile stresses. As a result, where fatigue loading is present it is desirable to eliminate or minimize tensile stresses by minimizing or eliminating bending and shear loading. Unfortunately, it is impossible to completely eliminate bending and shear since the orientation of the resultant load vector on the hip changes relative to any predetermined neck axis orientation during normal activity and thus it is impossible to completely eliminate bending and shear effects.
The hip joint reaction load varies in magnitude and orientation relative to a given reference frame fixed to the pelvis. Further, the joint reaction load varies in direction relative to a reference frame fixed in the femur. However, the major variation in the direction of the joint reaction load relative to the femur occurs in the femoral plane which is defined herein as the plane defined by the center of the natural or replacement femoral head, the distal tip of the femoral stem or the corresponding point on the proximal femur into which the femoral stem is implanted and the centroid of a cross-section of the most proximal portion of the femoral stem where such cross-section is taken perpendicular to the neck axis or a corresponding point on the femur were such prosthesis implanted therein.
Typical variation of the load in the femoral plane is illustrated in FIG. 1 where in this figure line A lies in a vertical plane during two legged stance. The angle .gamma. is defined as the angle measured from line A to the force vector component in the femoral plane and varies from 0.degree. during two-legged stance where the joint reaction load is less than one half times body weight to about 20.degree. during human activities which produce relatively high joint reaction loads which are on the order of several times body weight.
The stress condition resulting from a combination of bending and compression is shown in FIG. 2. The two loading conditions produce a combined stress state which can be determined by simply adding the stresses resulting from pure bending and pure compression loading individually. It may be seen that the bending stress increases the maximum value of the compressive stress resulting from the combined loading and also where bending is sufficient can produce tensile stresses as a result of the comb loading. Where shear is present, since magnitude of the stress is a function not only of loading but of orientation, one can always find an orientation where shear stresses are absent and this stress state consists only of tension and compression. These tension and compressive stresses are commonly called the principal stresses. Thus, one has an analogous case where one has tension, shear and compression loading.
Typically, the heads of femoral prostheses are truncated spheres as shown in the U.S. Patents noted above to Charnley, Treace and Muller terminating in a truncation plane which plane is defined herein as the plane including at least three points on the truncation edge of the spherical surface. Although the patent literature, for example U.S. Pat. No. 3,843,975 issued Oct. 29, 1974 to Raymond G. Tronzo and the above-noted Heimke et al. patent, shows drawings of spherical heads which do not terminate in a truncation edge, these drawings are merely patent draftsman or artistical representations of femoral stems and the head configurations shown do not represent actual embodiments of a true prosthetic femoral head. These patents generally relate to the stem of the femoral prosthesis and the head is depicted for the purpose of providing a complete drawing and obviously is not intended to represent an actual prosthetic femoral head. More particularly, the femoral head shown in the Tronzo patent is inaccurate since the intersection between what is apparently a round neck and a spherical head cannot possibly result in the curved line shown. Most actual femoral head embodiments result in a partial spherical head wherein the spherical segment is only slightly greater than a hemisphere as typified by those disclosed in the Charnley and Muller patents and which represent actual head neck embodiments; the drawings in these latter two patents are, in fact, drawings of commercially available prostheses. Such truncation can produce a situation as shown in FIG. 3 where the truncation edge E enters the acetabular cavity C of an acetabular bearing member A and results in the edge E rubbing over the acetabular bearing surface B. This situation produces a stress riser at the edge in the bearing B accelerating bearing wear. Further, such truncation of femoral heads can reduce the separation strength as described in the Biomedical Engineering Corp. TECHNICAL REPORT, "Technical Considerations in the Selection of a Femoral Endoprosthesis," Pappas, M. J. and Buechel, F. F., published 1982, as compared to the separation strength of femoral heads which are more fully spherical.
It is common practice in the prior art to provide stem-type femoral prostheses of a given size stem with various neck lengths, particularly extended neck lengths, which are intended to compensate on revision for shortening of the leg resulting from hip and/or femur bone erosion; revision being the term used by those skilled in the art to describe generally the replacement of a previously implanted prosthesis by another prosthesis due primarily to the malfunction or loosening of the previously implanted prosthesis. This situation is illustrated in FIG. 11 where due to hip bone erosion and/or removal of additional hip bone in revision, the acetabular cup prosthesis 30 which articulates with the head 32 of the stem-type femoral prosthesis 33 will occupy the position shown in dashed outline instead of the originally implanted position shown in solid outline. Such acetabular cup positioning upon revision causes what is referred to in the art as limb or leg shortening with the amount of shortening also being shown in FIG. 11. Leg or limb shortening is also caused by sinking of the femoral prosthesis 32 downwardly into the femur 35, and upon revision of the femoral prosthesis 33, it would occupy a lower position than is shown in solid outline in FIG. 11.
The most common prior art practice in use to extend neck length to overcome leg shortening is to use a common neck angle .alpha. and simply make the neck longer; such an extension is shown in dashed outline and superimposed on an originally implanted femoral prosthesis 37 which is to be revised and which is shown in solid outline in FIG. 12. As will be further noted from FIG. 12, the stem bending moment of the revision prosthesis 36 is increased with such a method of neck extension since the bending lever arm L2 is increased in length as compared to the bending lever arm L1 of the originally implanted prosthesis 37.
As shown in FIG. 14, using the method of neck extension described in the above-noted Treace patent to provide a constant head-stem distance between the lateral edge 41 or the stem axis 43 and the head center 44 of the extended neck prosthesis 45 and the head center 46 of the originally implanted prosthesis 47, the abductor muscle lever arm L4 (distance L2 between the line of action 40 of the abductor muscle and the head center 44 of the extended neck prosthesis 45) is reduced as compared to the anatomical or originally implanted abductor muscle lever arm L3 (distance D1 between the line of action 40 of the abductor muscle and the head center 46 of the originally implanted prosthesis 47) thereby reducing the effectiveness of the abductor muscle and increasing the joint reaction load.
The present invention, inter alia, as taught in detail below, provides a solution to this prior art leg or limb shortening-neck extension problem.